1. Field of the Invention
This invention relates to methods of and apparatus for sorting articles in accordance with both their thickness and resistivity. More particularly, this invention relates to methods of and apparatus for sorting semiconductor wafers in accordance with both their thickness and resistivity, wherein a pivotally mounted transitional track is provided for moving the wafers from a first track to a second track while a receiving position along the second track is activated by stored signals indicative of both the thickness and the resistivity of the wafers.
2. Description of the Prior Art
In the manufacture of electronic components such as integrated circuits, it is necessary for a number of different reasons to control both the thickness and the resistivity of semiconductor wafers used as substrates for the components. For example, it is necessary to control the thickness to make certain that the wafers have adequate strength to reduce breakage in the various wafer processing steps. It is also necessary to control the thickness in the manufacture of those components where impurity diffusion is carried out from both sides of the wafer, such as in the manufacture of silicon rectifiers. Another reason for controlling thickness is to make certain that excessive wafer material is not being needlessly and uneconomically used. In addition, by controlling the thickness of the wafers, large lots of wafers can be processed simultaneously through process operations where variations in thickness beyond a predetermined amount could cause anomalous failure of the process lot. Examples of these process operations are mechanical polishing, chemical polishing and photolithography.
In the manufacture of electronic components, it is also important to control the resistivity of the wafers for a number of reasons. For example, control of resistivity is required for the preparation of electrically isolated portions of wafers in the manufacture of integrated circuits. Also, since resistivity has a bearing on the depth to which impurities may be diffused and a bearing on the concentration gradient of the diffused impurities, it is necessary to control the resistivity of the wafers for these reasons.
In the past, thickness and resistivity have been determined separately and this has resulted in excessive handling of the wafers, with a consequent increase of sorting time and error, and of damage to the wafers. Also, in order to check resistivity, thickness had to be first obtained and the resistivity instrumentation has to then be manually adjusted in accordance with the thickness.
Accordingly, it is advantageous to sort the wafers automatically in accordance with both their thickness and resistivity. It is also desirable at the same time to decrease the possibility of damage to the wafers, and to decrease the sorting time and error.
It is also advantageous to sort the wafers using a simple and efficient apparatus which does not damage the wafers. For example, it is advantageous to use a gravity feed track and locate receiving stations along this track to eliminate the use of more costly air bearing tracks. This gravity feed arrangement also offers the advantage of compactness and reduces floor space requirements.